How Long Does It Take to Build an ADU? (Realistic Timeline)

Ask a builder how long an ADU takes and you'll hear "about six months." Ask a homeowner who just finished one and you'll hear "eighteen." Both are telling the truth about different things: the builder means construction, the homeowner means the whole journey from idea to keys. Here's the realistic timeline, phase by phase, and where it actually goes wrong.

Phase 1: Feasibility and design — 1 to 3 months

This is reading your city's rules, confirming what fits on your lot, and getting plans drawn. The range is wide because the route matters enormously. A pre-approved city plan can compress this to weeks; a custom design on an awkward lot can run three months or more with revision cycles. Prefab sits in between — the plans exist, but siting them on your specific lot still takes engineering.

Phase 2: Permitting — same day to 6 months

This is the phase with the widest spread in the entire process, and it's almost entirely determined by which city you're in and which plan route you chose. San José issues same-day permits for qualifying pre-approved plans. San Francisco's median permit timeline runs to roughly 186 days. Same state, same law, wildly different experience.

The nuance worth understanding: California law requires ministerial approval within 60 days of a complete application. But almost every submission comes back with corrections, and each correction cycle restarts the clock on that round. Two correction cycles is normal, not a failure. Plan for the realistic total, not the statutory minimum.

Phase 3: Construction — 3 to 8 months

A garage conversion typically runs 3–5 months: no foundation, no roof, less weather exposure. A detached new build runs 5–8 months, sometimes longer, because it's a complete small house with excavation, foundation curing, framing, and full mechanical systems. Prefab units compress on-site time dramatically — often to weeks — but add factory lead time and require crane access, which not every lot has.

Phase 4: Inspections and occupancy — 2 to 6 weeks

Final inspections, corrections, and the certificate of occupancy. Rarely dramatic, occasionally slow if a re-inspection lands in a busy week. Don't let anyone move in before the certificate — it can void insurance and complicate any future sale or refinance.

The realistic total

Add it up honestly and a garage conversion runs roughly 6–12 months end to end, while a detached build runs 10–18 months. If someone quotes you six months for a detached ADU from scratch, they're quoting construction only and leaving out the year of design and permitting around it.

What actually causes delays

In rough order of frequency: incomplete permit applications that trigger extra correction rounds; discovering site conditions after work starts (bad slab, unmarked utilities, poor soil); change orders — every mid-project decision to move a wall or upgrade a kitchen costs weeks, not just dollars; and contractor scheduling, since good ones are booked months out.

The pattern is that most delay is front-loaded and preventable. Time spent on complete plans and a thorough site assessment before submission pays back several times over in the phases that follow. Check your city's typical timeline in our city guides — the variation between them is larger than most people expect.

Run the numbers for your project

Every project is different — size, scope, and city move the total by six figures. Get an itemized estimate in under a minute:

Written and reviewed by ADUCostGuide. Cost figures reflect 2026 data and typical projects; rules and prices vary by city — see our city guides and confirm details with your local planning department and licensed contractors.